Conventionally, many proposals have been made for catalysts for polymerizing conjugated dienes such as 1,3-butadiene, isoprene, etc., and some of the proposals have been industrialized. For example, catalysts consisting of a combination of a transition metal compound such as titanium, cobalt, nickel, neodymium, etc., and an organic aluminum are often used for producing conjugated diene polymers having a high content of cis-1,4 structures.
Polymerization of a conjugated diene using an element of Group III of the periodic table as a catalyst is publicly-known, and various polymerization methods have been proposed so far. For example, JPH7-268013A (Patent Document 1) discloses a catalytic system consisting of a rare-earth metal salt, an organic metal compound of an element of Group I to III of the periodic table, and a fluorine-containing organoboron compound. JPH11-80222A (Patent Document 2) discloses a polymerization catalyst consisting of a compound of a metal of Group IIIB of the periodic table, an ionic compound of a non-coordinating anion and a cation, and an organic metal compound of an element of Group I to III of the periodic table.
As a polymerization catalyst using an yttrium compound, Pamphlet of International Publication No. 2006/049016 (Patent Document 3) discloses a catalytic system using an yttrium compound containing a bulky substituent group, and JP2007-161918A (Patent Document 4) discloses a catalytic system using an yttrium compound containing bis(trimethylsilyl) amide as a ligand. The content of cis-1,4 structures in conjugated diene polymers obtained using such catalytic systems is generally 97 to 98.5%.
As a catalytic system by which a conjugated diene polymer having a very high content of cis-1,4 structures can be obtained, JP2002-256012A (Patent Document 5), JP2004-027179A (Patent Document 6), etc. disclose catalysts that contain a metallocene-type cation complex of a rare-earth metal compound. However, a metallocene-type complex is difficult to synthesize and isolate, and hard to handle because it is very sensitive to air, water, etc. There is another problem; a conjugated diene polymer having a very high content of cis-1,4 structures can be obtained only at a low polymerization temperature.